[tool] Fix false positives in update-exceprts

script/tool/lib/src/update_excerpts_command.dart

@@ -206,20 +206,28 @@ class UpdateExcerptsCommand extends PackageLoopingCommand {
         .renameSync(package.pubspecFile.path);
   }
 
-  /// Checks the git state, returning an error string unless nothing has
+  /// Checks the git state, returning an error string if any .md files have
   /// changed.
   Future<String?> _validateRepositoryState() async {
-    final io.ProcessResult modifiedFiles = await processRunner.run(
+    final io.ProcessResult checkFiles = await processRunner.run(
       'git',
       <String>['ls-files', '--modified'],
       workingDir: packagesDir,
       logOnError: true,
     );
-    if (modifiedFiles.exitCode != 0) {
+    if (checkFiles.exitCode != 0) {
       return 'Unable to determine local file state';
     }
 
-    final String stdout = modifiedFiles.stdout as String;
-    return stdout.trim().isEmpty ? null : 'Snippets are out of sync';
+    final String stdout = checkFiles.stdout as String;
+    final List<String> changedFiles = stdout.trim().split('\n');
+    final Iterable<String> changedMDFiles =
+        changedFiles.where((String filePath) => filePath.endsWith('.md'));
+    if (changedMDFiles.isNotEmpty) {
+      return 'Snippets are out of sync in the following files: '
+          '${changedMDFiles.join(', ')}';
+    }
+
+    return null;
   }
 }

script/tool/test/update_excerpts_command_test.dart

@@ -232,11 +232,11 @@ void main() {
         ]));
   });
 
-  test('fails if files are changed with --fail-on-change', () async {
+  test('fails if READMEs are changed with --fail-on-change', () async {
     createFakePlugin('a_plugin', packagesDir,
         extraFiles: <String>[kReadmeExcerptConfigPath]);
 
-    const String changedFilePath = 'packages/a_plugin/linux/foo_plugin.cc';
+    const String changedFilePath = 'packages/a_plugin/README.md';
     processRunner.mockProcessesForExecutable['git'] = <io.Process>[
       MockProcess(stdout: changedFilePath),
     ];
@@ -253,6 +253,27 @@ void main() {
         output,
         containsAllInOrder(<Matcher>[
           contains('README.md is out of sync with its source excerpts'),
+          contains('Snippets are out of sync in the following files: '
+              'packages/a_plugin/README.md'),
+        ]));
+  });
+
+  test('passes if unrelated files are changed with --fail-on-change', () async {
+    createFakePlugin('a_plugin', packagesDir,
+        extraFiles: <String>[kReadmeExcerptConfigPath]);
+
+    const String changedFilePath = 'packages/a_plugin/linux/CMakeLists.txt';
+    processRunner.mockProcessesForExecutable['git'] = <io.Process>[
+      MockProcess(stdout: changedFilePath),
+    ];
+
+    final List<String> output = await runCapturingPrint(
+        runner, <String>['update-excerpts', '--fail-on-change']);
+
+    expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('Ran for 1 package(s)'),
         ]));
   });